1. Field
The present invention relates to hot water recirculation systems. More particularly, the invention relates to a method and system for reducing the “cold water sandwich effect” of tankless hot water heaters.
2. Description of the Related Art
High energy costs and Increased public awareness of global warming and other environmental concerns have encouraged consumers to conserve energy to both decrease their environmental impact and to lower their utility bills. Most individuals can make the greatest difference in their energy consumption by reducing home energy use. The U.S. Department of Energy estimates that water heating accounts for approximately 13% of a typical house's energy requirements. Conventional hot water heaters are inefficient because they heat and store hot water in tanks even when no hot water is being used. Such inefficiencies are commonly referred to as stand-by heat losses. The stand-by heat loss for a typical hot water heater has been estimated to be approximately 50%. Conventional hot water heaters also radiate heat which must be offset by increased air conditioner operation in the summer, further increasing utility bills.
Many consumers are therefore replacing conventional hot water heaters with tankless water heaters. Tankless water heaters directly heat water on-demand, as it is required, and have no storage tanks, thus eliminating stand-by heat losses. Tankless hot water heaters also can deliver hot water to a faucet, shower head, or other water fixture more quickly because they are much smaller than conventional hot water heaters and can therefore be positioned closer to the fixtures. When a hot water heater is positioned closer to a water fixture, there is less piping between the hot water heater and the fixture and therefore less cold water in the pipes that must be discharged before hot water reaches the fixture. Because tankless water heaters have no storage tanks, they can often be installed in cabinets adjacent to water fixtures, thus delivering hot water to the fixtures nearly instantaneously. This reduces wasted water and idle time waiting for the hot water.
Unfortunately, tankless hot water heaters suffer from a phenomenon called the “cold water sandwich effect.” This term describes the introduction of cold water into a fixture's hot water supply line during on/off operation of a tankless water heater. The cold water sandwich effect, when present, appears as a momentary drop in hot water temperature as it is discharged from the water fixture.
The cold water sandwich effect is inherent in all tankless water heaters and is a result of their operating principles. Tankless water heaters generally operate as follows: 1) when a hot water fixture is turned on, a water flow sensor in a tankless heater senses the water flow through the heater; 2) the flow sensor triggers the water heater's heating element when the flow rate exceeds a pre-set minimum flow rate; 3) the water flow and water temperature are monitored and used to adjust the heating element to maintain a desired output temperature; 4) when water flow ceases or drops below the minimum flow rate, the heating element is turned off. To maintain a safe ignition sequence for gas-fired tankless hot water heaters, steps 1 and 2 typically take up to 10 seconds. During this ignition sequence, a small amount of cold water flows through the water heater. Each time hot water usage is stopped briefly and then started up again, this ignition sequence is repeated, and a small amount of cold water passes through the water heater. Water exiting a tankless water heater that is cold then hot then cold again (or hot, then cold, then hot) forms a “cold water sandwich.”
One known solution for the cold water sandwich effect is shown in FIG. 1. A small recirculating system A is installed after the hot water output line B of a tankless water heater. The recirculating system circulates heated water through the hot water supply line C for fixtures D served by the water heater. The recirculating system A includes a small conventional water heater E with a storage tank, an expansion tank F, a return or recirculation line G, and a pump H for recirculating heated water from the small water heater through the hot water supply line and back to the water heater. The water heater E acts as a mixing tank and uses its heating element to heat the water exiting the tankless hot water heater. The water heater also offsets the heat losses from the recirculation line G. The water in the hot water supply line C is always kept hot (or warm) even when the tankless water heater discharges cold water due to the cold water sandwich effect. Although this method solves the cold water sandwich effect, it is not desirable because it requires installation of a water heater, expansion tank, and pump downstream of the hot water output of the tankless hot water heater. This prevents the tankless water heater from being installed right next to a water fixture. Such a system is also difficult and relatively expensive to install.
Another known solution for the cold water sandwich effect is shown in FIG. 2. A recirculating system H is installed so as to recirculate water through the tankless hot water heater itself so that the heating element of the tankless heater keeps the water in the hot water line L warm. The recirculating system H may include a small storage tank I installed on the hot water outlet J of the tankless water heater, a recirculation line K between the hot water line L and the cold water input M of the tankless hot water heater, and a pump N for recirculating water through the tankless hot water heater when no hot water is being used by the fixtures. The storage tank I acts as a mixing tank to blend cold water exiting the tankless water heater with hot water in the tank. This type of system relies on the tankless hot water heater to heat the water in the recirculation loop and can typically only be used if the hot water supply and return lines are a full ¾″ in diameter. There are two primary problems with this method. First, tankless hot water heaters have a minimum btu level at which they heat water and therefore often use more energy than is required to keep the water in the hot water line warm. Second, tankless water heaters are not designed to be operated for long periods of time at low flow rates because they require water flow to carry heat away from their heat exchangers. Warm water constantly circulating through the heaters at a low flow rate does not transfer enough heat from the heat exchangers, thus shortening their life.